Characterization of spun PMMA/UiO-66-NH2@PMMA thin films and their SPR sensing response to haloalkane vapors

Abstract

Immobilization of metal-organic framework (MOF) particles onto a gold substrate using spin coating is a difficult task due to the lack of cohesion forces between the crystals and the substrate. Therefore, it was decided to use polymethyl (methacrylate) (PMMA) as a matrix for MOF material. Spin coating was employed for the fabrication of PMMA films and crystals of an MOF [ $\text{Zr}_{6}(\mu {3}$ -O) ${}_{5}(\mu {3}$ -OH)5(NH2-BDC)6] ${}_{n}$ UiO-66-NH2 (NH2-BDC = 2-amino-1,4-benzenedicarboxylate) embedded in the PMMA (MOF@PMMA) films. PMMA and UiO-66-NH2@PMMA films were investigated using the surface plasmon resonance (SPR) spectroscopy to monitor film thickness, refractive index, homogeneity, and quality. These films were analyzed for their gas sensing capability toward selected haloalkanes, including dichloromethane (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4). The sensing properties were evaluated in the frame of a host-guest interaction. It was determined that the best response was obtained for CH2Cl2. This is interpreted based on the chemical structures and physical properties of the analyzed vapors. In addition, PMMA and UiO-66-NH2@PMMA films were found to exhibit fast response times (1-3 s) and selective character, indicating that they are efficient sensors for haloalkanes. The response of the MOF/PMMA mixed films compared with that of the PMMA was higher for CHCl3 and CCl4 vapors, whereas the incorporation of the MOF into the PMMA structure has improved the response times of the sensor for all vapors. To the best of our knowledge, this work represents the initial investigation of the gas sensing properties of the PMMA and MOF/PMMA mixed films via SPR technique for haloalkanes.